Pressure effects on CrCl6(3-) embedded in cubic Cs2NaMCl6 (M=Sc,Y) lattices: study through periodic and cluster...

The structural, elastic, vibrational, and optical properties of cubic elpasolites Cs2NaMCl6 (M=Sc,Y) containing CrCl6(3-) complexes have been investigated by means of both periodic and cluster calculations as a function of pressure in the framework of density functional theory. Aside from calculating the host lattice bulk modulus BH and the local modulus B1 associated with the CrCl6(3-), complex particular attention is paid to the pressure dependence of Huang-Rhys factors, Sa and Se (related to local a1g and eg modes), and the Stokes shift associated with the first electronic excited state 4T2g (t2g 2eg) of CrCl6(3-). The present calculations provide a big difference between BH=231 kbars and B1=676 kbars derived for Cs2NaScCl6:Cr3+ at zero pressure which plays a key role for a right interpretation of pressure effects on vibration frequencies and optical parameters due to CrCl6(3-). The significant decrease of Huang-Rhys factors, Sa and Se, due to the pressure observed experimentally is well accounted for by the present work which supports that partial differential Sa/ partial differential P is determined by the Gruneisen constant gamma a of the a1g local mode (whose frequency is nu a) and the dependence of 10Dq on the metal-ligand distance. At the same time, the present results point out that the Stokes shift would be little pressure dependent in the range of 0-50 kbars. Accordingly the Ham effect in the 4T2g (t2g 2eg) state of CrCl6(3-) in the cubic elpasolites would also happen for a pressure up to 50 kbars but the spin-orbit constant would increase with respect to that at zero pressure. From the analysis carried out in this work it is also concluded that the figures d nu a/dP=0.55 cm(-1)kbar and dSa/dP=-7.2 x 10(-3) kbar(-1) extracted from the complex emission band of Cs2NaScCl6:Cr3+ are hardly compatible. This fact underlines the usefulness of ab initio calculations for helping in the analysis of complex experimental findings. Finally, as the CrCl6(3-) unit is found to be to a good extent elastically decoupled from the rest of the elpasolite lattice, a model is shown to lead to an approximate relation between the pressure derivative of the local modulus and the Gruneisen constant gamma a.